1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus and an air-fuel ratio control method for an internal-combustion engine.
2. Description of the Related Art
An example of such an air-fuel ratio controller of an internal-combustion engine in the related art is disclosed in Japanese Patent Application No. 4,353,070. A catalyst is disposed in en exhaust channel in the internal-combustion engine to purify exhaust gas. A lean air fuel (LAF) sensor is disposed upstream of the catalyst, and an oxygen concentration sensor is disposed downstream of the catalyst. The LAF sensor linearly detects the air-fuel ratio in exhaust gas, whereas the oxygen concentration sensor detects the air-fuel ratio in exhaust gas and has an output characteristic that suddenly changes near an exhaust-gas air-fuel ratio corresponding to a theoretical air-fuel ratio.
This air-fuel ratio controller performs feedback control of the fuel injection such that the exhaust-gas air-fuel ratio detected by the LAF sensor reaches a target air-fuel ratio. A correction value for correcting the fuel injection in accordance with the exhaust-gas air-fuel ratio detected by the oxygen concentration sensor is calculated.
When the exhaust-gas air-fuel ratio detected by the LAF sensor is richer than the theoretical air-fuel ratio and the exhaust-gas air-fuel ratio detected by the oxygen concentration sensor is leaner than the theoretical air-fuel ratio, it is determined that a lean stuck defect in which detection values are shifted to the leaner side has occurred in the oxygen concentration sensor, and the upper limit of the correction value is set to a value smaller value. In this way, the correction value is prevented from being set to an excessive value when a lean stuck defect has occurred in the oxygen concentration sensor so that the fuel injection is appropriately controlled.